This invention relates to stress and fatigue control of cable, especially for titanium (ti) and aluminum (al) cable, while performing much greater amounts of useful work, primarily in materials handling systems. Two (2) additional patent applications are submitted herewith: (1) Fail-Safe Cable and the Effect of Non-Frangible Wire in Cable Structures, Ser. No. 757,551, and (2) Deep Well Handling and Logging Cable, Ser. No. 757,552. The three (3) patent applications are copending.
Three (3) physical phenomena: fracture, fatigue and wear occur to varying degrees in cable tension systems especially those in the broad materials handling category. These phenomena have dominant, and often progressive degrading effects, while to a much lesser extent, corrosion, stress corrosion, strain hardening and fretting fatigue normally have lower degrading affects while environments also have a pronounced influence. Because of titanium attributes, specially selected and processed titanium base alloy wire, assembled into titanium cable following suitable construction design criteria including novel counterbalancing, overcome or moderate these degrading effects when used in work performing tension systems. Al cable attributes are very similar in stress and fatigue control but in regard to these physical phenomena, results are different.
As examples of titanium wire and cable attributes, patent application file Ser. No. 707,951 relates to neutralization of wear while U.S. Pat. Nos. 3,527,044, 3,511,622 and 3,511,719 describe ti structural wire and cable and their uses, one being a process and one being an article patent.
Steel wire and cable are noted for rapid fatigue especially in worn and case hardened areas in the cable assembly. At points where the wire of one strand cross those of another strand at an angle, wire pressure and relative movement between wires cause sawing and case hardening. Under repeated bending, dynamic stressing under work load causes rapid wear and fatigue; notches wear in the wires at points of cross wire contact that leads to both wear and fatigue fractures, and progressive, and even rapid failure of the cable.
This physical notching characteristic with attendant case hardening and embrittlement, causes rapid fatigue under bending action. Conventional factors of safety in handling systems then range from 3 to 11 in practice, the latter being for elevators due to the premium that must be placed on personnel safety.
This safety approach while providing much more than adequate strength, increases cable stiffness, internal wear, and adds mass. The stiffness problem is overcome by using small (diam.) wires to provide flexibility, the essential system handling characteristic; but sliding friction and wire pressure between wires are increased because of increased mass and the high modulii of elasticity of wire and cable. Cable flexibility is then a function of wire size and elasticity as well as frictional stiffness; one is then concerned with both elastic and frictional stiffness in steel materials handling systems. Complex symmetry of massive old cable constructions degrades cable performance and specifically results in high internal stresses and low fatigue life.
Four (4) dynamic factors (stresses) in cable tension systems, known to induce abnormally rapid fatigue, are:
1. Impact stress (denoted by the formula, .sigma.i=.nu.o.sqroot.E.sub.c x.rho..sub.c) causes tension peaks sufficiently high to exceed the elastic limit so as to overstress the cable or some of the wires in the cable. Wire fractures, or overstressing (even cable fracture) frequently occur in high stress regions due to low dynamic properties of steel wire.
2. Bend stresses (denoted by the formula .sigma.b=(.delta./D).multidot.E) may be sufficient to cause plastic flow if the sheave or drum diameter is small enough.
3. Surface contact stress (.sigma.c=4580.sqroot.P) causes rapid fatigue because of the large pressure constant derived from dynamic factors of mass density (.rho.) and tension (T).
4. The total of characteristic dynamic stresses, when performing work especially including wire pressure and surface contact stresses, may be high enough to, cumulatively, cause fatigue from high tension peaks and even rapid fatigue from plastic flow denoted by wire notching or flattening. Severe impact stresses, also, may fracture the tension member as well as overstress it. Fatigue effects are severest in bend stress regions and near points of impact stresses of tension systems as shown in long cycle testing primarily in the lower stress region.
The cumulative values of dynamic stresses are calculated and tabulated hereinafter to show why rapid fatigue occurs in steel cables.
Continuous stress vibration of tension members, particularly notable in shock environments, adds constantly to fatigue accumulation. This vibratory fatigue is least in air, among the fluids, but may become quite severe in deep ocean steel moorings. Wire cross sections are markedly deformed by constant low order impacting from this vibratory phenomenon.
Strain hardening, as it operates in the plastic load range of wire, is known to fatigue steel to a much greater extent than ti upon which this phenomenon has little effect.